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“Nothing in Biology Makes Sense Except in the Light of Evolution”
(Theodosius Dobzhansky)1
Theodosius Dobzhansky (1900-1975) was one of the principal architects
of the so-called synthetic theory of evolution.2 Born and initially trained in Russia,
where he became an entomologist and zoologist with wide-ranging interests, he
brought a rich background in systematics and study of natural populations with
him when he came to the United States to learn genetics in the laboratory of T. H.
Morgan in 1927 (Adams 1994). In the end, he remained in the US, in part because
of political developments in Russia, where he was ultimately declared a nonperson. Dobzhansky utilized his double background of work with natural
populations and in Mendelian genetics in writing what came to be the single most
influential book in the formative period of the synthetic theory of evolution
(Dobzhansky 1937). Dobzhansky is a particularly interesting figure to study
because of the cultural dualities (or, rather, multiplicities) which he embodies,
because his work forcibly reconciled biological disciplines (systematics and
natural history on the one hand, genetics on the other) that had not only drifted
apart, but had built strong evidentiary bases for conflicting claims, and because,
like many of the founders of the evolutionary synthesis, he sought to relate
biological findings to issues in the wider culture.
This chapter takes its title from a talk that Dobzhansky delivered to the
American Association of Biology Teachers (Dobzhansky 1973a). The title of that
talk, “nothing in biology makes sense except in the light of evolution,” is often
cited both in controversies over the status of evolutionary studies within biology
and in various cultural debates over the (supposed) conflict between evolution and
religion. In this chapter I examine Dobzhansky’s claim and some of his arguments
in its favor with an eye to both of these contexts. I focus first on the setting, both
within biology and within American culture, of Dobzhansky’s arguments for the
importance of evolutionary biology. Second, I turn to the relations among
biological disciplines or fields, especially between evolutionary biology and
molecular genetics, a field that “took off” after the formation of the evolutionary
synthesis. Finally, I deal with some complications arising from the difference
between the intellectual and the institutional positions of the field of evolutionary
1
First published as Chap. 6 of The Epistemology of Development, Evolution, and
Genetics. The original version of this chapter was presented in 1995 at a colloquium in
honor of Marjorie Grene, “Conceptions de la science: hier, aujourd’hui et demain,” at the
University of Burgundy; the French version is published in Conceptions de la Science –
hier, aujoud’hui et demain : homage à Marjorie Grene, ed. J. Gayon and R. Burian
(Brussels: Ousia, 2007). The present version has been greatly improved by discussion at
the colloquium and the criticisms and suggestions of Jean Gayon and Marjorie Grene.
2
About the synthetic theory and some of the current debates about its status see, e.g.,
Burian (1988), Gayon (1990), Grene (1983), Grene (1990), and Mayr and Provine
(1980). For further biographical details about Dobzhansky, see Adams (1994), Ayala
(1990), Gould (1982), and Provine (1981).
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biology. I can, of course, do only partial justice to Dobzhansky’s advocacy of the
centrality of evolutionary studies within the biological sciences.
EVOLUTIONARY THEORY IN BIOLOGY AND EVOLUTIONARY BIOLOGY IN THE
UNITED STATES
A remarkable fact about evolutionary biology, given its importance in 20th
century biology, is that there have been virtually no departments of evolutionary
biology for most of this century (as there have been of genetics, for example).3 It
is important to start from the fact that Mendelian genetics and Darwinian
evolutionary biology were in serious conflict with each other in the first third or
so of this century. There are many aspects to this conflict, both intellectual and
institutional.4 I offer here a first approximation account of some of the differences
involved. Some of them follow from the practical necessity for early Mendelians
to work with mutations with large effects. Indeed, since at first all known
mutations were large in effect, the Mendelians held that such mutations were the
norm and provided the basis on which new species were formed. In contrast, most
natural historians and Darwinians insisted on the absence of large mutations in the
3
Interestingly, the major exception is Russia where, early in the century, there were
intense and productive scientific debates over various Darwinian and non-Darwinian
theories of evolution. At the end of the 1930s most major Soviet universities had
departments of Darwinism. But in consequence of Russia’s intellectual isolation in the
1930s, the controversies over Lysenkoism, and Lysenko’s ascent to power, they deployed
a distinctive Soviet version of Darwinism that did not match orthodox evolutionary
theories and practices in the West at that time. On this topic, see Adams (1991). See also
Provine (1980) for some useful material on the institutional bases for genetics. Provine
does not explicitly take up the formation of departments of genetics within colleges and
universities.
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There is a huge literature on the debates involved. Important references include Bowler
1983, 1988, 1989; Depew and Weber 1995, Part II; Gayon 1992, 1998; Provine 1971. On
this last topic, for a contrasting point of view see Nordmann (1994). In a recent paper,
Scott Gilbert (“Back to the Future: Resynthesizing evolutionary and developmental
biology,” presented at the Fifth Mellon Workshop on History and Social Sciences of
Contemporary Life Sciences, MIT, 1995) points out an important institutional aspect:
during the fifties, population genetics received considerable funding from the U.S.
Atomic Energy Commission in connection with concerns about the effect of radiation on
populations. As he puts it: “Whereas most evolutionary studies have difficulty getting
funds and students, concerns about the genetic effects of radiation enabled Dobzhansky
and others a constant supply of money and graduate students” (MS p. 5). On this topic,
see also Beatty (1991) and his “Opportunities for genetics in the atomic age,” presented
at the Fourth Mellon Workshop on History and Social Studies of Contemporary Life
Sciences, MIT, May, 1994. The lines of work thus funded thrived, but they did not have
significant impact on the organization of academic departments or on the larger
theoretical issues in evolutionary biology.
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field and on the gradual character of changes from one species to another. Thus,
early Mendelians and turn-of-the-century Darwinians agreed that the effects of
Mendelian mutations were too large to allow natural selection on Mendelian
mutations to control the features of organisms with any precision, including
features of organisms in a new species. Natural historians did not find significant
numbers of Mendelian mutants in natural populations; accordingly, the
Darwinians derided Mendelian mutations as experimental freaks that would be
eliminated immediately from natural populations and as irrelevant to evolutionary
history. Geneticists, in contrast, argued that Darwinians had no adequate causal
basis from which to explain the changes that took place in speciation and which
were marked by the striking visible differences between “good species.”
Turning to the institutionalization of biology, the term ‘biology’ came to
be employed rather self-consciously in some quarters around the turn of this
century for experimental biology. The insistence on experiment was used to help
legitimize a series of new sciences, genetics among them, as distinct from natural
history, botany, zoology, systematics, paleontology, and speculative Darwinian
evolutionism. Part of the point of excluding these old-fashioned sciences was that
they did not have adequate means of testing explanatory hypotheses or, worse yet,
they did not even offer genuine explanatory hypotheses. One aspect of the
movement toward experimental biology was the physics-envy of biologists and
their long struggle of biology to demonstrate that their science is as legitimate as
physics, a struggle in which Darwin himself had been engaged. Many participants
in the experimental movement deliberately excluded Darwinian theory from (their
kind of) biology precisely because it was ‘too speculative’ and had no means of
adequately controlling or testing its speculations (especially after the impact of
Haeckel and Weismann).5 The success of this ‘experimentalist’ movement is
marked internationally by the dominance of the new experimental disciplines in
the departments and research units begun after, say, 1910.
Although this is only an approximate account of one part of the deep
background to the matters of primary concern here, it usefully marks
Dobzhansky’s accomplishment. During the twenties and into the thirties, the
founders of mathematical population genetics, especially Fisher, Haldane, and
Wright in the Anglophone world,6 using models employing large numbers of
mutations with small effects, provided mathematical demonstrations that
Mendelism is compatible with Darwinian gradualism. Indeed, given enough
mutations of small effect, large enough populations, stable enough conditions, and
enough time to approach equilibrium, the mathematics demonstrated that the
5
Mark Adams forced me to recognize the importance of the experimentalist movement. I
am very grateful to him for many helpful discussions of this topic. A major debate on this
issue was triggered by Allen (1979) and a symposium on the topic edited by Jane
Maienschein, Ronald Rainger, and Keith Benson, Journal of the History of Biology 14
(1981), pp. 83-176.
6
Dobzhansky of course also knew the work of Chetverikov and Timofeeff-Ressovsky.
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selection would be the dominant factor in shaping the changing characteristics of
organisms within a population. At the same time, laboratory work in Mendelian
genetics had demonstrated that there are many more mutations with small effects
than mutations of large effect and that the mutations of small effect can modify
what is done by those with large effects. Thus the groundwork was laid for
reconciling genetics with studies of natural populations. But the relevant
communities of investigators were not sympathetic with one another and did not
read each other’s work. And there were certainly plenty of serious questions to be
asked about whether the assumptions of the models corresponded to the biological
realities to be found in natural populations and about the relevance of change
within populations to speciation. It was Dobzhansky’s virtue to bridge the
communities of naturalists and geneticists, to find a methodology for doing
experimental work with natural populations, and to write a book that so
formulated the issues that, after much dispute, both communities were, by and
large, persuaded.
In part on the basis of his extensive work with natural populations,
Dobzhansky had always held that there is an immense reservoir of variation
within populations in nature. In Genetics and the Origin of Species (1937), he
synthesized his findings and those of many others to provide an integrated
account, consistent with genetics, of the origin of species as an extrapolation of
the microevolutionary changes that the naturalists had described in detail. He also
argued that these results are entirely consistent with the models of population
genetics (particularly Wright’s). Furthermore, his account did not employ the
difficult mathematics of the population geneticists. In consequence, it made their
results accessible to both the larger genetics community and the natural historians,
who were largely unequipped to handle the mathematics (as was the case with
Dobzhansky himself). In the early papers of a series of 43 experimental papers on
the genetics of natural populations, the first 18 of which were published from
1938 to 1948 (Lewontin et al. 1981), he managed to show that natural populations
have a large reservoir of (mostly recessive) Mendelian mutations, that there are
significant genetic differences between local populations, and that the predictions
of certain of Wright’s mathematical models are compatible with the changes
found in the populations of Drosophila pseudoobscura that he studied.
It is worth noting a controversial problem on which Dobzhansky took a
strong stand. The issue stems from a paradox. The more powerful natural
selection is, the more likely it is to consume the variation that it requires in order
to change the characteristics of the organisms in a population. The reason is
simple: where there are selective difference among conspecific organisms within
a population, there is some likelihood of losing traits that are less advantageous in
favor of a single optimal solution to the physiological and ecological problems
faced by the members of the population (or the closest to optimal among the traits
that are available). The stronger the coefficients of selection, the more rapidly
alternative traits are lost. So how can there by high degrees of variation within a
population if the traits of the organisms in that population are shaped by
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selection? To the extent that the proponents of the synthetic theory, Dobzhansky
among them, hold that the dominant factor shaping organisms in evolution is
natural selection, to that extent they should expect variation to be minimized.
How can Dobzhansky’s insistence on the power of selection in leading to the
origin of species be reconciled with the high degrees of variation he found,
required, and advocated as the typical condition of natural populations?
Many geneticists, e.g., Herman Muller, who occupied the ‘classical’
position, applied this argument at the genetic level. They argued that selection
tends to reduce genetic variation, and that a population near equilibrium will be
largely homozygous, with only an occasional mutant allele here and there.
Dobzhansky, in contrast, advocated what came to be known as the ‘balance’
position, to wit that at equilibrium, most loci would be heterozygous, that there
were many more than two alleles available within the population at most loci, and
that the action of the genes will be balanced in such a way that the external
phenotype will be rather homogeneous, masking the enormous amount of genetic
variation within the population.7 To support this view, Dobzhansky argued that
the availability of different alleles enables organisms to respond successfully to
the wide range of environmental conditions that they encounter in their lifetimes.8
Putting it crudely, selection favors phenotypes capable of responding to a wide
range of humidities, temperatures, constituents in food, toxins, and so on, and to
adjusting developmental schedules to meet environmental conditions. The best
way to do this is to have (to use Waddington’s term) a canalized phenotype9 plus
high amounts of genetic variation, so that most individuals have two distinct
alleles available at most loci unless there is specific harm done by one of those
alleles.
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There is continuing controversy about the extent of justification for, and truth in, these
two polar positions. During Dobzhansky’s lifetime the epistemological difficulties
involved in resolving the dispute were insuperable; they remain severe even now. The
locus classicus for setting out the biological side of the epistemological issues is
Lewontin (1974). Lewontin’s analysis of the subsequent ramifications of the controversy
is controversial (see, e.g., Dietrich, 1994).
8
I have ignored some important shifts in Dobzhansky’s position over time. Two issues on
which his views changed concern the extent of coadaptation among the alleles
represented in a given local population (see Lewontin 1993) and the importance of the
effects of small population size on the variation within populations. (This factor made an
essential contribution in Wright’s models of evolutionary change.) As Dobzhansky
became more selectionist and the synthesis ‘hardened’ (cf. Gould 1983a), random
variation in small populations came to play less of a role in Dobzhansky’s and many
other synthetic theorists’ accounts of evolution.
9
This means, roughly, that the development of the organism is so programmed that it
lands in more or less the same adult (and intermediate) external phenotypes in spite of
wide variation in environmental conditions. For an early text setting forth this important
concept, see Waddington (1942).
6
This argument depends crucially on denial of the assumption that genes
are ‘the target of selection’ (a term due to Ernst Mayr).10 This strategy is natural
for those who study natural populations, for they understand selection in terms of
the differential consequences of possessing different phenotypes, but it is less
natural for geneticists, who calculate selection coefficients wholly in terms of
changes of gene frequencies.11 Curiously, Dobzhansky himself adopted the
formula that evolution equals change of gene frequency, a position for which
Mayr criticized him severely.
Turning briefly to larger contexts, two topics serve as emblems of the
larger situation in the US – the institutionalization of research support within
biology and the tendency in American society to see evolutionary biology as
standing in conflict with organized religion and with creationist doctrines. These
two issues are intertwined to a surprising degree. One way of showing the
connection is to reflect on the teaching of biology in US high schools. Stephen
Jay Gould did so in an essay written in December 1981, while attending a trial in
which a law from the state of Arkansas was being challenged. The law, ultimately
declared unconstitutional, required equal treatment in schools of ‘Creation
Science’ and ‘the Evolution Theory’ (supposedly equally justified and scientific).
Gould’s essay (Gould 1983b, chap. 21) descries the lack of evolutionary content in
the dominant high school biology textbook, from which both he and I were taught
in the 1950s. The book, Modern Biology by Moon, Mann, and Otto, was the lineal
descendant of Biology for Beginners by Truman J. Moon, first published in 1921.
Gould points out that the frontispiece of the original text was an image of Charles
Darwin and that the book was organized around the idea of evolution. He shows
that after 1925, the date of the infamous Scopes trial, the evolutionary content was
removed from the text. Not only is Darwin’s image gone, the very term evolution
is replaced by the cowardly paraphrase, ‘the hypothesis of racial development’!
The 1956 edition, for example, contains a brief chapter (the 58th of 60) touching
on evolution. The chapter evades the issue of creation vs. evolution by such
circumlocutions as these:
10
See, e.g., Mayr 1959, 1962, and 1963. It should be noted that the argument also
depends on assumptions about mechanisms of gene expression.
11
Mayr has repeatedly criticized Dobzhansky and others for defining evolution in terms
of change of gene frequencies. There are many additional reasons for resisting such a
definition, e.g., the way in which it underemphasizes the centrality of speciation (which
cannot be defined in terms of gene frequencies) in evolutionary change. The issue of the
extent to which natural selection can be understood primarily by means of calculations at
the genic level continues to be actively debated, e.g., in controveries over the genic
selectionism of R. Dawkins, G. C. Williams and others. See, e.g., Dawkins (1976, 1982;
Williams (1966). There are a number of important connections to some issues about
biological hierarchies – see, for example, Grene 1987, especially the relations of
ecological and genealogical hierarchies and various debates over the level(s) of selection.
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During these ages [of geological change], species of plants and animals
have appeared, have flourished for a time, and then have perished as new
species took their places.... When one race lost in the struggle for
survival, another race appeared to take its place.
This is how high school biology was taught to my generation. This sad
fact reflects the political power of biblical fundamentalism in the US and the
country’s genius for intellectually unsatisfactory compromise. Against this
background, is it any wonder that there was no impetus, even among professional
biologists, to place evolution at the center of biology curricula or to organize
biological disciplines so as to ensure that evolutionary studies played a key role in
the organization of biological work in general? Dobzhansky was keenly aware of
this situation and faced it head-on in the address whose title stands at the head of
this chapter.
Although it is not simple to demonstrate, it is extremely plausible that this
cultural background had an important influence on the structure of professional
biology. Scott Gilbert, for one, connects the strength of creationism to the
structure of the NSF: “In the United States, evolution is still so suspect that no
National Science Foundation study section is designated evolutionary biology.”
Difficult as it is to make a direct connection to the cultural milieu in such a
matter, I believe that there is widespread consensus among evolutionary biologists
that this has been a central problem in the institutionalization of their (would-be?)
discipline. The contrast with genetics, which receive major public and private
funding in virtue of its economic importance, is striking.
DOBZHANSKY’S ARTICLE12
Dobzhansky begins his article by discussing the rejection, on scriptural grounds,
of the Copernican theory of the heavens as a ‘mere theory’, not a ‘fact’ by a Saudi
fundamentalist, Sheik Abd el Aziz bin Baz. Dobzhansky lays out a very standard
argument against this view in a Darwinian style, arguing that the hypothesis that
the earth orbits the sun “makes sense of a multitude of facts that are otherwise
meaningless or extravagant.” Dobzhansky is ready to grant, for the sake of the
argument, that no direct observations demonstrate the ‘fact’ that the earth orbits
the sun, but insists that we must dismiss the geocentric alternative as making
nonsense of many facts we have observed. Similarly, there are no direct
observations of the age of the earth or of the evolution of new species, but an
enormous multitude of facts show both the diversity and the unity of life that
would be meaningless or extravagant except in the light of evolution.
Rhetorically, Dobzhansky makes excellent use of the parallels between the
pro-Copernican and pro-Darwinian arguments and the fact that the antiCopernican fundamentalist is a Muslim from Saudi Arabia. By adding the cultural
12
Throughout this section, page numbers refer to Dobzhansky (1973a).
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distance between a Saudi fundamentalist and mainstream Americans to the
strongly held belief in a heliocentric solar system in the US, Dobzhansky makes it
difficult for a biblical literalist to contest the pro-Copernican argument.
Dobzhansky argues that the literalist is put in the position of blaspheming if he
treats the Koran or the bible as a primer of natural science, for when scientific
developments outrun the literal reading of scripture, the literalist is forced to deny
well established fact or to make God into a deceiver by planting misleading
evidence.13 Beyond that, he argues, scripture is meant to deal with far more
important matters, “the meaning of man and his relation to God,” matters that
require the use of poetic symbols comprehensible both to the people of a given era
and to people from other times (p. 125). In the main body of the paper,
Dobzhansky spells out the parallel argument illustrating at some length the
multitude of facts in biology (but also geology, radiometric dating, and much
more) that evolution is required to explain.
Of particular interest here is Dobzhansky’s treatment of molecular
biology. The diversity of life is quickly evident from the sheer number of species
and bodily diversity among plants, animals, and microorganisms, the
extraordinary diversity of the ecological niches they occupy, and the narrowness
of the adaptations that many organisms exhibit. It is somewhat harder to establish
the unity of life. Dobzhansky treats molecular findings as the most decisive, but
by no means the only, evidence for the unity of life. (He also cites, e.g., traditional
evidence of homologies and of shared developmental pathways from comparative
anatomy and embryology and illustrates the ways in which diversity can come
from unity by the remarkable radiation of drosophilid flies on the Hawaiian
Islands.14) Molecular evidence for the unity of life includes the universality of
DNA and RNA as the genetic materials, the universality of the genetic code and
the mechanisms by means of which the code is translated into sequences of amino
acids, the high uniformity of cellular metabolism (cf. the roles played by such
compounds as adenosine triphosphate, hemes, riboflavin, pyridoxin, etc. in nearly
all organisms), and the consistency of the rate of substitution of amino acids into
such highly conserved proteins as the cytochromes. Such
13
For example, in dealing with the massively coherent radiometric evidence of the age of
the earth, dismissed as ‘mere theory’ by “Sheik bin Baz and his like,” one must ask
“What is the alternative? One can suppose that the Creator saw fit to play deceitful tricks
on geologists and biologists. He carefully arranged to have various rocks provided with
isotope ratios just right to mislead us into thinking that certain rocks are 2 billion years
old, others 2 million, while in fact they are only some 6,000 years old” (p. 126).
14
We now know that over 800 of the world’s more than 2000 species of drosophila,
including some of the most aberrant ones, are found on a geologically isolated landmass
3% the size of France. Because of their volcanic origin, the islands have well-defined
ages; the oldest is less than 6 million years old, the youngest less than 1 million years.
The immense diversification and speciation of a founding stock must have taken place in
that time.
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biochemical or biological universals ... suggest that life arose from
inanimate matter only once and that all organisms, no matter how diverse
in other respects, conserve the basic features of the primordial life. (It is
also possible that there were several ... origins of life; if so, the progeny
of only one of them has survived and inherited the earth.) But what if
there was no evolution, and every one of the millions of species was
created by separate fiat? However offensive the notion may be to
religious feeling and to reason, the antievolutionists must again accuse
the Creator of cheating. They must insist that He deliberately arranged
things exactly as if his method of creation was evolution, intentionally to
mislead sincere seekers of truth (p. 127).
The claimed centrality of evolution within biology was meant both for
biologists and for a larger public. This is demonstrated by the whole tenor of
Dobzhansky’s career. To cite but one piece of evidence regarding the larger
public, one need only look at the books he published aimed at a larger audience.15
Within biology I note his constant efforts to place all biological questions within
an evolutionary framework and to ensure an institutional footing (and funding) for
evolutionary studies. It is a minor, but indicative, symptom that the phrase
“nothing in biology makes sense except in the light of evolution” serves as the
epigraph of a co-authored textbook on evolution, published posthumously
(Dobzhansky et al. 1977).
MORE ON THE RELATIONS AMONG BIOLOGICAL DISCIPLINES
I now leave Dobzhansky to examine some complexities concerning the
relationship between evolutionary studies and molecular biology. There is
considerable irony in the interactions between the two, especially with regard to
the stance that Dobzhansky and other leading ‘non-molecular’ evolutionists took
in favor of the centrality of evolution within biology. I will speak to the relevant
developments briefly and abstractly, first in terms of the relationships among
biological disciplines, institutions, and funding, then in terms of some issues
about the molecularization of biology.
An interesting way of thinking about the history of biology in the
twentieth century is in terms of the founding of moderately autonomous
disciplines and their subsequent interactions. Thus, as indicated in the first section
of this chapter, early Mendelian genetics was fairly sharply separated from
(Darwinian) evolutionary biology. An enormous gulf also arose between genetics
and embryology. The two disciplines employed different techniques, came to
adopt conflicting experimentally grounded presuppositions (e.g., concerning the
nuclear vs. cytoplasmic origin or control of key traits of multicellular organisms),
15
The list includes Dobzhansky 1956, 1963, 1964, 1967, 1972b; Dobzhansky and Dunn
1946; and Dobzhansky, Boesiger, and Wallace 1983. Dobzhansky’s activity along these
lines includes a considerable amount of periodical literature and editorial work as well.
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and employed different organisms (with strikingly different properties) for the
exemplary research on which disciplinary consensus was built.16 Specialization
became ever more important in the course of the century, and it brought with it
the price of significant barriers among biological disciplines, both within the
experimental tradition and between experimentalists and naturalists. There were,
of course, always those who sought to integrate the findings (and occasionally the
techniques) of different specialties, but this was no simple matter. It is, of course,
the job that Dobzhansky believed could be accomplished by evolutionary biology.
But insofar as the institutionalization of biology is concerned, as already
indicated, no firm institutional setting was developed to serve as a base for
evolutionary biology as an integrative discipline, at least into the 1980s.
The roots of molecular biology (which are currently much disputed),17 are
to be found in transmission genetics, crystallography, information theory,
biochemistry, the application of physical techniques from physics, and a number
of allied sources, none of which are centrally interested in evolutionary questions.
I have argued in a number of articles (as have many others) that molecular
biology is best understood as a loosely interrelated collection of tools, wedded to
the ideal that one can elucidate and clarify biological function by analyzing
molecular structures.18 To the extent that this characterization is fair, at least as a
first approximation, the problems of evolution do not belong to the initial core
problems of molecular biology although, of course, someone interested in
evolutionary questions can apply the new molecular tools to them – something
that has been done with a vengeance in recent years with the development of a
major (sub?)discipline of molecular evolution and the flourishing of evolutionary
studies within a number of the specialties of molecular biology. Nonetheless, as I
will argue in the concluding section of this chapter, the pathway by means of
which the development of studies of molecular evolution came into being was
neither straightforward nor obvious.
MOLECULAR BIOLOGY AND EVOLUTION
There are two aspects to the difficult birth of molecular evolutionary studies. One
is the weak institutionalization of evolutionary studies in general. The other
16
On this topic, see chap. 10 below or Sapp (1987). For one embryologist’s view of the
threat posed by genetics to his discipline see HarrisoN (1937). For my views on some
more recent developments, see chaps. 11 and 12 below.
17
The broadest standard history is Judson, 1996 (1979). For an unusual position on the
proper delimitation of molecular biology, see Zallen (1993). Michel Morange provides a
useful synthesis of conflicting views on this topic, including a criticism of Zallen’s point
of view in Morange (1994, 1998).
18
See, for example, the symposium edited by Soraya de Chadarevian and Jean-Paul
Gaudillière on “The tools of the discipline: Biochemists and molecular biologists,”
Journal of the History of Biology 29 (3) (1996): 327-462. I state some of the points at
stake here more forcefully in Burian (1996).
11
concerns some aspects of the intellectual history of early work in molecular
biology. Let me take these one at a time. Institutionally, as I have argued above,
there were few natural bases for evolutionary biology, especially in connection
with molecular biology, rooted as it was in the experimentalist traditions that
tended to exclude evolution as a serious topic of study. As I did not argue
explicitly, early molecular biology drew heavily on biochemical and molecular
studies and other lines of work initiated by the ‘invasion of the physicists,19 In
spirit, it was experimental and mechanistic. It required experimental
demonstration of the specific action of particular genes or physical-chemical
influences, such as particular radiations, chemical mutagens, and metabolites. One
characteristic of the new generation of molecular biologists was the drive toward
quick and decisive experimental resolution of experimentally tractable questions.
This meant that they favored the use of microorganisms in virtue of their
experimental tractability and the rapidity of the experiments they permitted.
Indeed, studies based on microorganisms displaced slower evolutionary projects
both in funding and in the post-war institutionalization of biology. Studies of
long-term evolutionary scenarios were looked on unfavorably, whereas short term
mechanistic questions, for which rapid techniques were available (cf.
ultracentrifugation, electrophoresis, use of radioactive tracers, plating of phages),
came to dominate molecular genetics.
Work on the mechanisms of molecular action displaced work with
traditional genetic and evolutionary organisms (and was favored with better
funding) in part because of the obvious potential for applications to human health
and improvement of agricultural production. The diversion of funding to these
purposes tended to displace more general evolutionary studies. The reshaping of
departments of biology in the post-war years followed the same trends.
Accordingly, in many leading institutions, evolutionists felt that they were
fighting rearguard actions to retain their positions and prestige in the face of the
expansion of molecular biology and the indifference of most molecular biologists
to evolutionary issues.
One particular consequence of the concentration on microorganisms in the
early days of molecular biology is particularly ironic. Since microorganisms
(especially viruses and procaryotes, organisms with no true nuclei and relatively
limited amounts of DNA) experience enormous pressure for economy in the use
of genetic material, they are under much greater pressure to find optimal solutions
to environmental problems than are eucaryotes (nucleated organisms, mostly
multicellular, with excess DNA buffered by a variety of protective devices).20
Again, because of lack of buffering of the genome, there is a tendency for nonfunctional genetic variation in microorganisms to be more readily eliminated than
19
Two classical sources on this topic are Fleming 1968 and Judson 1996 (1979).
Including diploidy; many of the microorganisms studied were haploid and so each and
every allele was required to function properly for survival, a claim that is not true for
diploid organisms.
20
12
in eucaryotes.21 As a result, many geneticists who worked with phage and bacteria
were prepared to infer that the organisms they studied were ‘perfectly adapted’22
or very nearly so. What this means is that microorganisms were perceived as
having eliminated genetic variants that interfered with optimal performance in
physiological conditions. In this respect, they do not carry obvious stigmata of
their history; rather, adaptation dominates over historical pathways. Thus the early
findings in microbial genetics tended to find relatively less variation that
Dobzhansky expected and, thus, to support the notion of ‘perfect adaptation’ at
equilibrium, and with it the classical, rather than Dobzhansky’s balance theory.
These conclusions were, of course, resisted by those evolutionists who worked
with natural populations, especially populations of diploid eucaryotes. Such
naturalists had long since developed evidence for the presence of large amounts of
covert variation in populations of eucaryotic organisms.
It was only in the mid-1960s, with the adaptation of the biochemical
technique of gel electrophoresis to population studies of multicellular
eucaryotes,23 that is, with the development of techniques that could, and did,
detect enormous amounts of variation in the amino acid composition of particular
proteins in natural populations, that the molecular techniques began to address the
question of the amount of variation in diploid populations and, thereby, indirectly,
the potential for evolutionary change in seemingly monomorphic populations. The
21
Among the many reasons for this, eucaryotes are typically diploid. Many genes that
would be lethal if they were present in a single copy or when homozygous, are harmless
or even beneficial, when heterozygous because they are protected by being paired with a
functionally normal allele. Because of interactions among genes and gene products, the
heterozygotes containing such “lethal” genes can even be beneficial, as in the example of
sickle cell anemia. Again, eucaryotes often have highly evolved systems of gene families.
Multiple copies of physiologically important genes allow some copies to evolve silently
since other copies of the gene perform the required function. Similarly, other features of
eucaryotic genomes can increase the protection of a particular gene from the immediate
action of selection. For example, the separate pieces of “interrupted genes” in eucaryotes
(which have separated units, often corresponding to functional parts of the corresponding
protein) can be duplicated independently of one another, and combined with pieces from
other genes. This allows parts of genes to be stabilized and recombined, and to evolve
independently of one another. Yet again, eucaryotes often do a great deal of "posttranscriptional modification" of messenger RNA. That is, they have multiple means of
processing genetic information after it is transcribed onto RNA and before it is
“translated,” i.e., decoded to yield a protein. These devices allow evolution of the
modulation of a genetic signal without a direct change in a particular gene. There are
literally hundreds of complex phenomena that could be listed here.
22
For development of the importance of the doctrine of perfect adaptation in Darwin’s
work and the struggle he had to go through to replace it with the weaker notion of relative
[sufficient, but imperfect] adaptation, see Ospovat (1981). For one digest of the
importance of this shift for the evolutionary synthesis, see chap. 4 above.
23
The breakthrough papers, which set off an entire industry, were Hubby and Lewontin
(1966) and Lewontin and Hubby (1966).
13
results led to immense and continuing controversy. However, to a first
approximation it is safe to say that they supported Dobzhansky’s balance against
the classical position at least in this respect, that they showed there to be
immensely more variation present in natural populations than could readily be
accounted for on the classical hypothesis. Precisely how these findings affect the
debates about the power of natural selection and what bearing they have on the
paradox caused by the tendency of natural selection to consume the variation that
is relevant to the formation of an optimum remains a matter of controversy to this
day.24 This is a story that we cannot follow here, but one that allows us to bring to
closure this brief study of the status and influence of Dobzhansky’s stance about
the importance of evolution within biology.
CONCLUSION
During the last twenty years, molecular biology has produced a series of
extremely startling findings. As was once said by astronomers of the system of the
heavens, if God had consulted the biologists, they would have been able to devise
for Him simpler ways of building the systems that maintain life. In particular,
organisms are built of a variety of pieces and systems that were not designed to fit
together. As is now well known, this is true down to the molecular scale. Genes of
eucaryotes are built in pieces that have to be spliced together, the intermediate
products encoded by those genes are sometimes altered as much as six or seven
times at six or seven different stages of construction, both within the nucleus and
after being exported into the cytoplasm.25 In many organisms, mitochondria
(organelles required for respiration in air) employ slightly variant genetic codes
than the rest of the cell. This list could be expanded indefinitely. Suffice to say
that the complications of the machinery out of which organisms are built are
nothing short of incredible. And these complications bear the stigmata of a
complex history. Systems of control that turn on the machinery for making eyes in
mice have recently been shown to turn on the machinery for making eyes in fruit
flies.26 Yet the sorts of eyes that are made, and the steps by which they are made,
are entirely different. The only sensible understanding of this mixture of
conserved control systems and novel constructional apparatus is that the control
system is a historical remnant, one that has for reasons we do not understand very
well, been retained for over 500 million years. There can be no pretense of perfect
adaptation here. Organisms are cobbled together. To use François Jacob’s word,
24
The locus classicus for discussion of this debate is Lewontin (1974). See also Dietrich
(1994).
25
For amplification on these topics see chapters 9 and 11, and for a far stronger account
of the nature and impact of recent findings, see chapter 12.
26
Halder, Callaerts, and Gehring 1995. Chap. 11, below, was an early reaction to the
findings of the Gehring laboratory.
14
they are constructed by a process of bricolage (roughly, tinkering). This bricolage
means that the different parts of organisms and even cells (cf. mitochondria) have
different histories. Careful examination of the current features of organisms thus
reveals a great deal about the contingent pathways by means of which they were
constructed. The importance of this deep truth within biology was central to
Dobzhansky’s perspective. To this extent, the last two decades of work in
molecular biology have helped to justify his claim: nothing in biology makes
sense except in the light of evolution.
15
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